Friday night at a “spaghetti dinner” in Tilton, New Hampshire, a no-nonsense woman named Lisa Smith Brunelle, who is a single mother of four kids either in college or recently graduated, asked Mitt Romney whether he had any ideas for reducing the 8% interest rate they are paying on their college loans. Romney said one idea would be to increase the money available to write off college costs in exchange for joining the National Guard. “My son considered that,” Ms. Brunelle told me later, saying he’d looked at the Marines in particular. But he decided against enlisting because his cousin went to Afghanistan to pay for college, and a year ago Thursday he stepped on an IED, losing a leg, Ms. Brunelle said.

With Gov. Mitt Romney surging in South Carolina, and leading by 23 points in New Hampshire ahead of next Tuesday’s primary here, it may take a series of gaffes by the candidate for any of his GOP opponents to catch him. They don’t have much to hope for in that respect. Romney built his lead in part on the strength of his largely mistake-free performances in the Republican debates of the last six months. And between campaign stops this past week, Romney has been preparing with his staff for the debates tonight and tomorrow.

But now and then you catch a glimpse of gaffability in Romney. About once during an appearance, his normally rigid discipline gives way to a kind of goofy spontaneity, which in turn can produce a potentially damaging misstep. Usually the errors are harmless, and even when they aren’t, he’s quick-witted and self-aware enough to find his way out. The biggest problem for Romney is that his blind spots tend to be for sensitive subjects, ones with potential resonance for an economically struggling and war-weary electorate. Which means an uncaught gaffe could be very costly indeed.

The first category where Romney tends to get into trouble is money, in particular, his. (Remember the $10,000 bet?) At an event Wednesday evening in Peterborough, New Hampshire, for example, Romney, whose net worth is in the neighborhood of $200 million, went into an apparently unplanned but lengthy discussion of whom he wanted to leave his money to. Previously, he said, he’d told his estate lawyer he just wanted to leave it all to his sons, not his grandkids. But now, he said, he wants to leave it all to his grandkids, not his sons. If the story was supposed to make a point—the estate tax is bad, maybe—Romney realized he was in trouble and pulled the plug before he made that argument. Just the fact, though, that he spent what seemed like an eternity talking about the disposition of his multi-million dollar estate must have set off his strategists’ alarm bells.

The second area where Romney gets into trouble is lack of empathy. He’s plenty sympathetic to the woes of those who attend his events. But where he seems acutely aware, even excited, about businesses or market dynamics–no doubt a good thing at a time when Americans are desperate for renewed prosperity–he can seem out of touch with the concerns of middle class Americans. And sometimes the contrast jumps out.

Romney seemed “a little shocked by the amount we’re paying,” Ms. Brunelle later said of his response to the interest rate part of her question, bringing to mind George H. W. Bush’s costly lack of familiarity with the price of a gallon of milk during a debate with Bill Clinton and Ross Perot in 1992. And coming from a man who was born wealthy and received a deferment from the draft, Romney’s cheerful suggestion that Ms. Brunelle’s kids consider joining the military to pay for college might have fallen flat even without knowing the sad coincidence of her nephew.

Most of the time mistakes like these cause no damage. Romney’s estate tax misstep was quickly forgotten in an otherwise successful post-Iowa celebration in Peterborough. Ms. Brunelle, whose nephew is recovering well, she says, has no hard feelings for Romney about either the interest rate or the National Guard suggestion, and plans to vote for Romney in both the New Hampshire primary and the November election. But if his increasingly hard-pressed opponents want to try and take Romney down in coming debates, their best hope may not be attacking him frontally but rather getting him to talk about his money or other people’s problems.